Commercially available pressure sensitive adhesive tapes are usually provided in a roll form on a tape dispenser (see e.g. U.S. Pat. Nos. 4,451,533 and 4,908,278). Tape dispensers typically have either a metal or plastic serrated cutting blade. “Severability” of adhesive tape is defined as the ability to cut or sever a length of tape by pulling the tape over the teeth on the serrated cutting edge of a tape dispenser with a desired amount of energy or work. Severability is also referred to as “dispensability.”
It is desired that the severed tape does not chip, sliver, fracture or break in an unpredictable manner (see U.S. Pat. Nos. 4,451,533 and 4,908,278). Such severability is desirable to produce a cleanly serrated cut edge on the severed tape strip. Severability is governed primarily by the mechanical properties of the backing of the adhesive tape. Cleanly serrated edges are preferred for aesthetic reasons in applications such as gift wrapping, mending, and the like. The ease with which an adhesive tape can be severed depends on the deformation and resistance to break (toughness) of the tape backing film, also referred to as the substrate. Typically, the substrate is coated or laminated with surface layers to provide an adhesive surface or a matte or writable surface. The greater the energy required to sever the film, the more extensive the resulting damage to the film will be and the less aesthetically appealing the resulting severed edge. In most cases, energy to sever of the tape is governed primarily by the substrate, with little affect by the adhesive or other layers or coatings. This is believed to be due in part to the elastic strain energy built up during the deformation being suddenly and catastrophically released when the film's inherent strength limit is surpassed. The subsequent failure propagates uncontrollably in a tearing or ripping fashion so that the cut edge does not follow closely the contour of the teeth of the dispenser. The propagation also is directed along the underlying fibril orientation distribution rather than closely following the contour of the dispenser teeth.
The majority of commercially available biaxially oriented polypropylene film is produced by the flat film or tenter stretching process. Typical tenter processes serve to biaxially stretch films either predominately simultaneously or predominately sequentially. Sequential tenter stretching is currently the most widely used biaxial film processing method. Typically, a thick sheet is extruded and rapidly quenched to form spherules having α-morphology with a monoclinic unit cell. This thick sheet is then reheated to a suitable stretching temperature and stretched in a first lengthwise or longitudinal stretching step, followed by a second transverse stretching step and then the film is annealed to produce a flat film having uniform thickness. Simultaneous tenter stretched films comprise a minor part of the film backing market because, although such processes can continuously stretch films in both longitudinal and transverse directions, they have historically proven costly, slow, and inflexible regarding allowable drawing ratios.
Commercially available biaxially oriented polypropylene films are well known for their toughness, moisture stability, good color, slivering resistance, and clarity and have long been used as adhesive tape backings (see U.S. Pat. Nos. 3,241,662 and 3,324,218). Such films are produced typically with so-called “film grade” isotactic polyproylene resins having high molecular weight and low melt flow rates between about 2–8 grams/10 minutes as measured using ASTM D 1238-95.
High molecular weight polyolefin resins are well known in the art to produce biaxially oriented film having uniform stretch and thickness, without hardbands, wrinkles or other defects. High molecular weight imparts cohesive strength to the cast sheet, allowing uniform stretching at the strain rates typical of commercial biaxial film stretching equipment, especially the high strain rates that can occur in sequential biaxial orientation film lines.
However, especially in the sequential biaxial orientation case, high molecular weight produces film having high toughness, and adhesive tapes produced from such films tend to elongate significantly before breaking while under load, which renders such tapes extremely difficult to sever, particularly on a plastic-bladed dispenser. The elongation to break for such tapes results in an undesirably large amount of work on the part of the user.
It is known to add other components to polypropylene resin to improve processability or improve severability for production of adhesive tape backing films.
Small amounts (less than about 10% by weight) of low molecular weight additives like waxes or lubricants can be employed to allow the cast web to be more easily stretched to a film by reducing the internal friction between polymer chains as they slide past each other. WO 97/46369 discloses use of a polypropylene wax additive, by which the execution of a process for preparing a biaxially stretched polypropylene film at high speeds and with high reliability will be possible.
Components like hydrocarbon tackifiers, high Tg polymers and the like can be added to enhance the film brittleness and thereby reduce the work required to sever the film. Such materials may prove difficult to process or impart undesirable stiffness, opacity, off-colors, and higher cost to the finished film.
JP 53-34834 discloses a biaxially oriented polypropylene based adhesive tape with cutting properties formed from a polymer mixture containing 20–80% of polypropylene and 80–20% of a low molecular weight polyolefin. It is further disclosed that in the case when the molecular weight of the low molecular weight polyolefin becomes greater than 20,000 grams/mole, it becomes a material where the cutting properties are significantly deteriorated.
U.S. Pat. No. 3,887,745 discloses a finger-tearable adhesive tape with a film base comprising a layer of polypropylene polymer film which is biaxially oriented and at least one other layer of a polypropylene polymer film which is uniaxially oriented in a transverse direction, the melting point of the uniaxially oriented film being 0.5–10° C. higher than the melting point of the biaxially oriented film, and the total thickness of the uniaxially oriented film being 1.2–5.0 times greater than the total thickness of the biaxially oriented film.
U.S. Pat. No. 4,393,115 discloses a laminate film having hand-cutting properties comprising a biaxially oriented polypropylene layer and a uniaxially oriented polypropylene layer oriented only in the width direction, laminated with a stretched thin surface layer of propylene-ethylene block copolymer. The thickness of the uniaxially oriented layer is in the range between about 1.2 and 5.0 times the thickness of the biaxially oriented polypropylene layer.
U.S. Pat. No. 4,414,261 discloses a severable polypropylene adhesive tape having a base sheet comprising an intermediate layer made of crystalline polypropylene containing from 25 to 35% by weight of a petroleum resin and outer layers made of a crystalline polypropylene laminated on both sides of the intermediate layer, the thickness of the intermediate layer is within a range from 60 to 90% of the base sheet.
U.S. Pat. No. 4,447,485 discloses an adhesive tape with finger-tearability and severability comprising a base sheet of a polypropylene resin containing from 5 to 50% by weight of a methylpentene polymer and a subsidiary layer formed on one or both sides of the main layer composed of crystalline polypropylene having a melting point of at least the melting point of the methylpentene polymer, and a bonding layer of a carboxylic acid-modified polypropylene.
U.S. Pat. No. 5,474,820 discloses a multilayer polypropylene film comprising a base layer of polypropylene and at least one outer layer containing a mixture of HDPE and one or more olefin homopolymers, copolymers or terpolymers and the film has a silk-matte finish. The polypropylene polymer of the base layer has a melting point of at least 140° C. and the melt flow index is in the range of 0.5–15 grams/10 minutes.